Repetitive low energy seismic source system controlled by digital timing means

ABSTRACT

Initiation, in sequence, of pulses of seismic energy from a selected depth within a shothole using elongated charges of explosive gas mixtures, is provided by a seismic source system including novel digital data responsive control circuitry controlling the initiation and duration of gas delivery as well as ignition of the gas in accordance with control signals generated using, as a source code, clock pulses and digital signals representative of multi-bit timing words, such as, for example, a nine- or 21-bit digital word.

United States Patent Carruth, Jr.

[ Aug. 29 1972 [54] REPETITIVE LOW ENERGY SEISMIC SOURCE SYSTEM CONTROLLED BY 3,496,530 2/1970 Brown et a1. ..340/ 15.5 DP 3,133,231 5/1964 Fail et a]. ..l8l/0.5 XC

DIGITAL TIMING MEANS 3,460,648 8/1969 Brown et al. ..340/ 15.5 DP

[72] Inventor: Henry Thomas Carruth, Jr., Prim), rE-xaminer Benjmn Borchelt Houston Assistant Examiner-James V. Doramus [73 Assignee; Chevron Research Company, San Attomey-A. L. Snow, F. E. Johnston, R. L. Freeland,

Francisco, Calif Jr. and H. D. Messner [22] Filed: Dec. 21, 1970 57 ABSTRACT pp 99,946 Initiation, in sequence, of pulses of seismic energy from a selected depth within a shothole using elon- 52 11.5. CI. ..1s1/0.s xc, 340/155 DP gated charges exPlsive gas mixtures is Pmided by 51 Int. Cl. ..Glv 1/08 G01v 1/28 a seismic system including digital data [58] Field of Search mlsllo R o 5 3 05 responsive control circuitry controlling the initiation 3 2 5 and duration of gas delivery as well as ignition of the gas in accordance with control signals generated using, as a source code, clock pulses and digital signals [56] References Cited representative of multi-bit timing words, such as, for

UNITED STATES PATENTS example, a nineor 21-bit digital word.

3,058,540 10/1962 Simpson ..340/12 R 4 Claims, 18 Drawing Figures l I lSEJFJE I DISCHARGE /79 cIRcuI'r l i I l HIKI g CHARGE 6 I RELAY E l l M K I 20 i z M L I 1 FIRE FROM 1 R E L A Y F l E LD L GEOPHONES g a9 {L -l- 5 DIGITAL DIGITAL 01 FIELD SYSTEM CONTROLLER CHARGE 31? sass.

PATENTEmuszs m2 3.687.229 sum 07 or 12 HE/V Y TWA? CARRUTH, JR. M94 bw P'A'IENTEDwszs 1912 INVENTOR HEN Y THO A5 CARRUTH, JR.

PATENTEDmszs mm 3.687 229 sum 11 or 12 INVENTOR m w) THWJR.

BY WM 1 REPETITIVE LOW ENERGY SEISMIC SOURCE SYSTEM CONTROLLED BY DIGITAL TIMING MEANS BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to a low energy gas seismic source system and, more particularly, to repetitive seismic source operative by data digital control circuitry, for the initiation of the delivery of elongated charges of explosive gas mixtures to a selected depth within a shothole followed by ignition of gas charges to generate pulsed seismic energy repetitively.

In accordance with the present invention the digital data responsive control circuitry controls duration of deliverence of the gas charges as well as time (occurrence) of ignition of each charge in accordance with a digital time source code generated at a remote location, as in a recording truck, and includes, at the shooting truck, time decoding means for decoding the time code so as to control first and second relay means connected to the delivery means and the ignition means of the system. Remotely located time code generating means can also include gate means conditionally connected to time break generating means of the seismic source which enables the gate means for a selected time interval after the seismic energy has been initiated in the shothole. In that way, digitizing and recording equipment within the recording truck can be energized to digitize and record seismic reflections of the generated seismic energy.

2. Description of the Prior Art In geophysical seismic prospecting, a seismic wave is initiated at or near the earths surface and travels downwardly into the earth formation. After encountering interfaces formed between different subsurface strata, a portion of the energy is reflected back towards the earths surface. As is well known, after the reflected energy has been detected, travel time of the wave can directly be related to the depth of the reflecting interfaces.

Heretofore, dynamite has been used to generate seismic waves, usually by detonation in relatively shallow shotholes drilled into the earth. The depth at which detonation occurs is normally from 50 to 100 feet, so as to preferably be below the surface layer which is commonly referred to as the weathered layer of the formation. In that way, problems associated with wave propagation within the weathered layer are significantly diminished (normally the seismic wave within the weathered layer propagates at a much lower velocity than that in formations beneath the weathered layer).

Adjacent to the shothole, a geophone spread can be positioned in a particular orientation with respect to the source points to receive the seismic reflections in a manner allowing later enhancement by conventional seismic enhancement techniques, such, for example, as

common depth point stacking (CDPS)." (In CDPS, 6o

ticoverage of the same subsurface area, using the same shothole, at the same detonation depth, the shothole itself can become mechanically altered.

That dynamite is not the only method of generating seismic disturbances is well known. Repetitive (analog controlled) gas seismic sources are the subjectof US. Pat. Nos. 3,055,450 and 3,058,540 in which repeated ignition of a combustible gas mixture within a vertically oriented pipe carried aboard a boat is employed to produce repetitive seismic impulses for surveying earth formations overlaying bodies of water. With a lower or remote end of the pipe emersed in water, the mixture of gases is usually ignited by ignition means connected to the pipe producing a wave front which proceeds in a downward direction through the pipe and strikes the surface of the water at a velocity substantially in excess of the velocity of sound to generate seismic impulses. However, since the pipe is emersed in water, heat generated within the pipe during operation is usually rather quickly dissipated. Further, the resulting mechanical vibration due to the impact of the detonation wave striking the water, is likewise dissipated since the boat at the surface of the water is itself cushioned against severe vibration.

Additionally, since both the shooting and recording can be performed aboard a single boat, coordination of these operations is greatly simplified. However, on land, these functions are usually carried out at separate geographical locations, requiring the use of complex synchronization circuitry at both the shooting and recording sites. Where digital recording of the resulting seismic reflection wave is contemplated, for example, as shown in US. Pat. No. 3,496,530 in which the seismic disturbance is generated by a series of hydraulically activated vibrators, synchronization becomes further complicated. Additionally, if the source of seismic energy is explosive gas charges to be ignited, repetitively, in a shothole below the earths surface by means of digital control signals, both longand shortlived emergencies can, from time to time arise at the shooting truck. Under such conditions coordination becomes, obviously, even more difficult.

SUMMARY OF THE INVENTION In accordance with the present invention, a gaschargeable digitally controlled repetitive seismic source is located within a shothole penetrating the weathered layer of the earth formation and operative by novel digital control circuitry within a shooting truck in accordance with a selected digital timing source code generated by a more remotely located repetitive timing logic circuitry as within a recording truck. The source code comprises: digital signals representative of multi-bit binary timing words (say nineor 21-bit) as well as regularly occurring clock pulses.

Elements of the source controlled by the source code include:

I. an elongated pipe adapted for support within the shothole, the shothole being at least partially filled with liquid;

2. an elongated, enlarged firing chamber attached, at one end, to a remote end of said pipe within a shothole and open at the other end so as to contact the liquid within the shothole, the liquid within the firing chamber being used to form an impact surface for the chamber;

3. lateral oflset means connected to the near uphole end of said pipe;

4. means connected to the lateral offset means for delivering to the firing chamber through said lateral offset and said pipe, a combustible gas mixture of predetermined amount;

5. ignitor means mounted in said lateral offset means;

6. means electrically connected to the ignitor means for energizing the ignitor means to initiate the combustion of the mixture whereupon a combustion wave travels through said pipe and said firing chamber and strikes, at impact, the interior surface of said liquid to generate a seismic disturbance;

7. length/diameter ratio of the pipe being within a range to support travel of said combustible wave at supersonic speed; and

8. a firing chamber having a tapered end section connected to the pipe to reestablish the combustion wave thereinafter emerging from the pipe.

Digital data responsive control circuitry at the shooting truck preferably connects in parallel with the delivery means and the energizing means whereby sequential control of one or the other is provided, as by means of (a) a first relay means electrically connected to a time decoding means for decoding the digital source code and operative thereby to control the delivery means so as to cause delivery along the elongated pipe and firing chamber of a predetermined amount of gas mixture, and (b) a second relay means electrically connected to the decoding means and operative to control and cause energization of the ignitor means to cause ignition of the gas mixture after delivery.

To allow the operator at the shooting truck to possess emergency control functions, the time decoding means at the shooting truck includes a separately operative switch network means which can be activated, independently, by the shooting truck operator should emergencies arise. Shooting operations can be restarted at the same or adjacent sequence when the operation was stopped without repeating previously performed steps, or can be begun anew. Each shooting cycle can also be automatically repeated (up to 999 times) without operator intervention.

Elements of time decoding means include:

a. control means comprising start/stop/restart signal generating means and switch network means controllably connected thereto capable of generating digital signals for initiating, stopping and restarting delivery of gas and energization thereof in accordance with the state of the switch network means;

b. timing counter means connected to control means (a) responsive to digital signals therefrom and including additional switch network means, for accumulating clock pulses from a source of said pulses to provide a plurality of digital signals in accordance with switch conditions of the additional switch network means,

c. control signal generating means including gate means selectively responsive to digital signals from control means (a) and timing counter means (b) as well as to digital signals indicative of a multi-bit digital data word passing through control means (a) for generating at least first and second group of control signals in which said first group sequentially controls the first and second relay means of the seismic source,

d. cyclic record counting means including yet additional switch network means as well as gate and counter means responsive to digital signals from control means (a) which indicate each shooting cycle and compare the indicated cycle to an encoded ending cycle value in accordance with conditions of the yet additional switch network means, the record counting means ((1) also including a digital signal generating means responsive to digital signals from the gate and counter means thereof as well as to said second group of control signals generated by control signal generating means (c) to provide a cycle re-initiation digital signal, and

e. reset logic means including a series of gate means responsive to at least said cycle re-initiation digital signal from record counting means (d) to provide a series of set and reset digital signals for circuitry (a)-(d) for setting said (a)-(d) circuitry to a preselected initial condition which the seismic shooting cycle can be continually repeated without interruption until the ending cycle value encoded in record counting means (d) is attained.

Remotely located timing logic circuitry for generating the aforementioned time code may include gating means responsive to a time-break signal created by means of time-break indicating means attached to the firing chamber when a detonation wave passes through the firing chamber, whereby the gating means is held in an enabled state for predetermined duration to assure detection of all significant seismic reflection signals from the earth formation under survey, after generation of the seismic disturbance.

Other objects and advantages of the invention will become more evident from the following detailed description of a single embodiment when read in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a side elevation of an earth formation illustrating a series of shotholes penetrating the weathered layer of the formation, several of the shotholes each depicting a firing chamber of a seismic source connected uphole to a straight delivery pipe terminating at the earths surface, while a single shothole adjacent to a shooting truck is seen to accommodate a fully operational repetitive gas charge seismic source system in accordance with the present invention;

FIG. 2 is a side elevation illustrating in more detail, the fully operational seismic source system of F IG. 1 in which the firing chamber and delivery pipe are attached to a lateral offset pipe extending from the rear platform of the shooting truck;

FIGS. 3a, 3b, 4a and 4b illustrate details of the lateral offset pipe of FIG. 2;

FIG. 5 is a block diagram of a digital data responsive control circuit housed within the shooting truck of FIG. 2 for controllable delivery and ignition of gas mixtures to the firing chamber within the shotholes of FIG. I

using a preselected digital time code generated by timing logic circuitry remote from the shooting truck as within a separate recording truck;

FIG. 6 is a block diagram of remotely located digital field system including timing logic circuitry for use in conjunction with the digital data control circuitry of FIG. 5;

FIG. 7 is a detailed diagram of a digital data control circuitry of FIG. 5;

FIG. 8 is a digital diagram of the time decoder of FIG. 7; and

FIGS. 9-14 depict specific details of the time decoder of FIG. 8.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to FIG. 1. In FIG. 1 a series of shotholes 10a, 10b, 10c and 10d are depicted, each shothole over a region thereof being filled with a liquid 11 and extending below the earths surface 12 to a substantial depth below the boundary or innerface 13 between weathered layer 14 and sub-weathered strata 15 of the earth formation under survey. In gathering seismic data in the field, so as to provide multiple surface coverage of the same area many times, repetitive gas seismic source system of the present invention generally indicated at 17 is energized, in sequence, within each shothole 10a-10d. In this regard, to obtain sufficient seismic data, each shothole may be used as a source point many times, the shooting crew associated with shooting truck 16 thereafter moving the seismic source system 17 from shothole to shothole as required.

In gathering data in the field so as to provide subsurface coverage of the same area many times, not only may a multiplicity of records be made utilizing a repetitive seismic source, but is also may be desirable to simultaneously advance both the position of the seismic source as change in the inline position of the geophone spread occurs. For example, the geophone spread could be advanced an inline distance equal to the distance X between shotholes. In that way, the resulting seismic information provides multiple coverage of the same surface area many times.

In such a technique, rapid movement of the source system 17 along the inline direction of survey obviously increases the efficiency of the collection process. Towards this objective, the series of shotholes 10a-10d can be drilled by one crew in advance of the shooting and recording parties. A second crew can then position a series of source sub-assemblies, say six or more roughly 360 ft. apart, each sub-assembly comprising a firing chamber 18 located at depth within a shothole connected to a vertically elongated delivery pipe 19 terminating adjacent to the earth s surface 12.

After the shooting and recording parties have collected seismic data utilizing the aforementioned source subassemblies, a third crew can retrieve the subassemblies and transfer them, say in sets of six each, to forward position of the survey. However, it is not uncommon that several source sub-assemblies may have to be abandoned. A portion of the shothole 10a-10b can cave in resulting in a wedging of a firing chamber 18 within the shothole as depicted for shothole 10b in FIG. I.

FIG. 2 illustrates shooting truck 16 in more detail. As shown the truck 16 is provided with a mixing, delivery and ignition system indicated at 20 interior of housing 21 at its cantilevered rear platform 16a. Mounted ad- 5 jacent to platform 16a are a pair of fuel cylinders 26 and 27. Each cylinder 26, 27 is separately connected to the mixing ignition and delivery system 20 as by way of fuel inlet conduits 28. Within housing 21, these conduits become merged into a single pipe, as explained hereinafter, the single pipe eventually exiting from housing 21 as exit pipe 29, thence coupling to offset pipe 30. Opposite to the end connected to housing 21, the offset pipe 30 is placed in engagement with the uphold end of pipe 19 through coupler 31. In turn, the opposite end of pipe 19 connects to firing chamber 18 through a second coupler 32.

In operation, the mixing, ignition and delivery system 20 is actuated by a novel digital data control circuit shown in phantom line interior of truck 16 at 22, for delivery of a controlled amount of gas from the cylinders 26 and 27 through lateral offset pipe 30 thence to vertical delivery pipe 19. Since generation of the seismic disturbance requires that a liquid surface is established interior of firing chamber 18, a series of coextensive slots 33 are provided at the terminal end of the firing chamber. Preferably, the series of slots 33 have longitudinal axes aligned with the axis of symmetry of the shothole. Liquid 11 within the shothole enters into the interior of the firing chamber 18 through the slots 33 establishing an initial liquid level at line 34. The gas mixture interior of vertical pipe 19 impinges on liquid 11 initially at level 34 and thereafter causes relocation say to a level indicated by phantom line 35 in the interior of firing chamber 18. Thereafter the gas flow is terminated. The resulting Lshaped" column of combustible gas within the offset pipe 30, vertical pipe 19 and firing chamber 18 is then ignited to establish detonation wave. Within vertical pipe 19, the detonation wave reaches a velocity in excess of the speed of sound since the length to diameter ratio is controlled to be in the range of 10 to 80, with a range of 20 to being preferred. As the detonation wave strikes the surface of the liquid at surface 35 interior of the fir ing chamber 18, a seismic disturbance is created. In this regard, the firing chamber 18 is seen also to have a tapered entryway 36 increasing from a diameter identical with that of vertical pipe 19 into an increased diameter so as to accommodate a large volume of mixed gas. Since the detonation wave after passing through vertical pipe 19 must be reestablished within the firing chamber 18, tapered entryway 36 provides a transitional closure to accommodate the flame front across the full cross-sectional area of the chamber 18.

Attention will now be directed to the constructural features of couplers 31 and 32 at the ends of vertical pipe 19. As shown in FIG. 3a, firing chamber 18 is threaded for mounting to collar 37. Collar 37 in turn is threadably mounted to pipe 19 but with an opposite helical orientation. Accordingly, it is evident that coupling the firing chamber 18 and pipe 19 using opposite helical thread orientations allows for the uncoupling of the vertical pipe 19 (and collar 37)- downhole-from the firing chamber 18 should an emergency arise. The retrieving crew can thus easily uncouple the firing chamber 18 from collar 37 as, for

example, should firing chamber 18 become wedged within the shothole by simply rotating the pipe 19 at the earths surface in proper direction to effect release.

As shown in FIG. 3b, at the opposite end of vertical delivery pipe 19 coupler 31 is seen to include a collar 39 interiorly threaded to engage the pipe 19 at one end and, at the other end, is provided a series of basses 41 for engagement with offset pipe 30. Engagement is provided by rotating collar 39, as by bar 42, until bosses 41 contact annulus 43 attached to the offset pipe 30. To prevent leakage of the gas mixture from the pipes in operation the collar 39 is also provided, over a central portion, with an O-ring 44.

Consideration will now be given to the coupling of offset pipe 30 relative to shooting truck 16. As indicated in FIGS. 4a and 4b, the offset pipe 30 is not formed of a straight section of pipe but is provided a laterally displaced swivel joint 45 adjacent to the truck. The swivel joint 45 includes, at its ends a pair of collars 46 and 47. Collar 46 connects to exit pipe 29 of the mixing, delivery and ignition system 20 of FIG. 2 while collar 46 connects to the main section of offset pipe 30 through a short stub 49 and valve 50.

Between the couplers 46 and 47 are elbows 52 and 53. Relative rotation of the elbows 52 and 53 about axis A-A is provided by ball bearings (not shown) internal of central housing 54. An O-ring (not shown) seals the elbows relative to the housing 54 during such rotation. In order to simplify design, swivel joint 45 is preferably of conventional design. (An example found to be adequate is a swivel joint manufactured by the Aeroquip Corporation, Barco Division, Harrington, Illinois, Series 5200). In operation, the swivel joint 45 allows for rotation of the offset main section of pipe 30 about axis A-A in response to reaction forces. After the seismic disturbance is created within the shothole, upwardly directed forces act on the pipe 30. However, these forces can be dissipated at the swivel joint 45 without undue mechanical damage to the system, through angular rotation of the offset pipe 30 in which axis of symmetry B-B thereof tilts relative to the earths surface.

Valve 50 is merely for additional safety. It is hand operated by means of handle 55, as indicated in FIG. 4b, and provides for manual shutoff of the gaseous mixture as the recording truck is moved from shothole to shothole.

FIG. illustrates mixing, delivery and ignition system in more detail. As indicated in FIG. 5, the system 20 is externally connected as follows:

i. by an entry piping network to fuel cylinders 26 and 27 by way of fuel inlet conduits 28, ii. by an exhaust piping network to firing chamber 18, in party, by way of exit pipe 29, and iii. electrically to timing logic circuit 24 of digital field system by way of digital control circuitry, or controller 22. At its interior, the system 20 includes an elongated T- mixing chamber 64 having separate entry ports 65, 66 connected to the fuel inlet conduits 28. (For convenience of description, the mixing chamber 64 in FIG. 5 is shown in partial section while the electrical circuitry is shown in more schematic form). T-mixing chamber 64 includes first and second horizontal bores 67 and 68; bore 67 connects at one end to the ports 65 and 66 and at the other end to horizontal bore 68. Since ignition of the gas mixture is initiated within the horizontal bore 68, bore 68 is referred to as the ignition bore; while horizontal bore 67 connected thereto is referred to as the mixing bore. Oxygen enters the bore 67 at port 66 through cylindrical nozzle 69 provided with a side wall of diameter C terminating in an oblique pathway 70. Oblique pathway 70 is formed by terminating the side wall along a bias relative to the axis of symmetry of the bore 67. Propane enters the bore at port 65 by way of threaded coupler 71. Since the diameter D of the bore 67 is just slightly greater than the outside diameter of the nozzle 69, say by the amount that is twice the spiral interval d the propane entering the bore 69 at port 65 is obviously to be mixed with oxygen in non-equal proportions, say five parts oxygen to one part propane. Since the initial direction of the propane is at right angles to axis of the nozzle 69, impingement at its outer periphery causes, initially, diffusion, and later, spiral motion. Likewise, the oblique orientation of the pathway 70 at the opposite and of the nozzle 69 provides similar spiralling motion about the axis of the bore 67 for the oxygen. Mixing of the fuels is thus assured.

Horizontal bore 68 is seen to be larger in diameter than bore 67 and is provided at one end with sparkplug 74 and, at the other end, terminates in exit pipe 29.

Such construction does not mean, however, that during ignition of the gas mixture within the bore 68 that gas within bore 67 may not ignite; e.g., the flame front may enter bore 67 at junction 76 and propagate towards the nozzle 69. However, because of the fact that the horizontal bore 67 is perpendicular to the ignition bore 68, such condition is minimized, providing a first state flame arrest." Additionally, the relatively small spacing d between the outer periphery of the nozzle 69 and the side wall of the bore 67 provides for second stage arrest in the vicinity of the nozzle 69.

Consideration should now be given to the means for igniting the gas mixture within ignitor bore 68. As shown, sparkplug 74 is electrically connected by way of ignition coil 78 and a transistorized capacitive discharge circuit 79 to a source battery 80. Control of the transistorized discharge circuit 79 is by way of firing relay K interconnected between the discharge circuit 79 and the battery 80, the relay K being controlled by digital data controller 22 electrically connected to digital field system 25. As previously mentioned, digital data controller 22 is conveniently located within the shooting truck and is in electrical communication with timing, logic circuit 24 of the digital field system 25, the latter being usually located at a remote position therefrom, as in a recording truck through a series of conductors generally indicated at 81. The digital controller 22 not only controls fire relay K but also controls, on alternate time sequencies, a charging relay K, for entry of gas into the mixing chamber 64. As indicated, the relay K, connects to valves 82 and 83. On command, the relay K, is actuated to open the valves 82 and 83. On command, the relay K, is actuated to open the valves 82, 82 to gas flow under pressure. In this regard, pressure regulators are preferably included in the conduits 28, say at 84, 85, to maintain the oxygen and propane at selected pressure levels, say 50 psi for the oxygen and 20 psi for the propane. After a predetermined time interval as determined by the controller 22, the relay K is deenergined. Thereafter, the firing relay K, is activated, causing sparkplug 74 to fire through the transistorized discharge unit 79 and source 80. In this regard, since the source 80 is preferably a l2-volt standard battery, the discharge unit 79 can be readily purchased commercially, as a unit. After ignition of the gas mixture within the system 20, the firing relay K, is deenergized and a new cycle is initiated in accordance with a preselected repetitive rate.

Repetitive rate of the system obviously is dependent upon several operational factors such .as shooting depth, fuel pressure, heat conduction characteristics, etc. Once the rate has been established-programmed-at the digital field system 25, operations can occur continuously.

However, should the rate be initially. established at too high a level excessively high temperatures could occur adjacent within system 20, say at temperature condition thermalswitch 86 attached to exit pipe 29 of FIG. 5. As a new cycle occurs, gas entering the overheated section of pipe 29 can undergo combustion even though the sparkplug 74 is in a disabled state. Under such conditions, therrnalswitch 86 actuates switch contacts therein so as to independently interrupt the firing/charging cycles of the system, as by connecting fail-safe valves 87 and 88 directly to source battery 80 to cause closure. Although such interruption can be independent of both the digital controller 22 as well as the digital field system 25, it is preferred that thermalswitch 86 also cause energization of relay 89 for interrupting operation of digital controller 22. However, operation of the digital field system 25 need not be interrupted.

FIG. 6 illustrates digital field system 25 in more detail. Prior to the discussion, in detail, of the operation of digital field system 25, it should be evident from the prior description of FIG. 5 that digital system 25 provides two main control functions:

i. delivery and ignition of gas mixtures in sequence to cause a series of disturbances in accordance with a preselected timing code; and

ii. digitization and recording in binary format received reflection seismic signals.

With reference to item (ii), above, an enabling signal indicative of zero time for recordation of seismic signals is required. For this purpose, the present invention provides time-break switch 90 at firing chamber 18 of FIG. 2. As each detonation wave passes through the chamber 18, time-break switch 90 closed to initiate an enabling signal to the digital field system 25 of FIG. 6. In more detail, digital field system 25 of FIG. 6 is seen to include timing logic circuit 24 previously mentioned including a timing logic circuit 91 under partial control of gate switch means 92. At the closure of time-break switch 90 a one shot multi-vibrator within gate means 92 is initiated for a predetermined time period, say 4 to 6 seconds. During the time interval, reflection seismic signals can be received and passed by way of data input conductors and amplifiers 93 for eventual digital recordation at magnetic tape unit 98. ,In more detail, the reflected signals pass from amplifiers 93'to multiplexer 94, thence to analog-to-digital converter 95, master copy logic circuit 96, formal control circuit 97 and finally onto magnetic tape unit 98. As the data is received at amplifiers 93, binary gain shifts are indicated by binary gain feedback control circuit 99 and the master copy logic circuit 96. To provide data processing controls compatible with computer processing techniques, logic circuits 99 and sequentially operate utiliiing timing and word counters 101 and 102. As is conventional, the timing and logic circuit 23 produces a series of timing (clock) pulses to establish a time code applied to the logic circuits 96 and 100 through word counter 101 and the block counter 102. Accordingly, received seismic signals can be correctly identified, digitized and then recorded onto the magnetic tape at magnetic tape 98 inproper sequence.

Recordation of the'seismic signals received on amplifiers 93 first requires the determination of the' am- 'data, the gain is then gated through the binary control logic circuit'100 in proper sequence to themaster copy logic circuit 96 for digital recording on thesame channel as the binary seismic data. At multiplexer 94, the amplitude of each analogs signal is electrically sampled, in sequence, over a plurality of very small time intervals, say, 0.002-second intervals. These signals are then transferred to digital converter 95 where" the digital result of the multiplexing operation is represented by a series of multibyte binary code indications. The binary code information is electrically suited for storage on magnetic tape at the magnetic tape unit 98 on the same channel as its associated binary gain in? formation.

During allthese steps, all activity is paced by regularly occurring clock signals, as through master clock 103. Each operation of master copy logic circuit 96 requires a certain number of clock pulses. Con-.

sequently, timing to complete any one of the various operations is an exact multiple of the clock pulses as determined in part, by word and block counters 101 and 102. However, operation of counters 101 and 102 are not limited to digital recording of seismic data. The timing logic circuit 24 through block counters 101 and 102 can provide enabling signals for th e'digital controller 22. I

FIG. 7 illustrates, in detail, digital controller 22. As indicated, controller 22 includes time decoder 1. .5 comprising a series of gates-and flip-flop circuits, as explained below, for time dependent, controlled energization of relays K, and K, as through transistors 106 and 107. Each transistor 106, 107 is independently operative. For example, the presence of sets of enabling signals at the time decoder can enable, independently, one or the other transistors 106,107 to allow current to pass from power supply 108 by way of the relays K, or K thence through the transistor 106 or 107. As indicated, power supply 108 is connected to l2-volt battery source 109. Interconnected between the power supply 108 and the relays K K is a diode dampening circuit 110. Dampening circuit 110 dam- DETAILED DESCRIPTION OF TIME DECODER 105 FIG. 8)

Reference is now made to FIG. 8 in which time decoder 105 is shown in detail. Within the recording truck, the time break signal (TB) serves to initialize its operations to produce a time source code, used, in turn, to synchronize operations of the shooting truck. Although the number of recording cycles, or records, may be repeated automatically at an arbitrary value, say up to 999, the operation of time decoder 105 remains a function of the time source code generated by the digital field system (DFS) 25 of FIG. 5.

As shown in FIG. 8, input terminals 1 a-1 10d of the decoder 105 pass DFS signals 82-8 as follows:

i. terminal 110a passes the work and block pulses comprising a part of the source code through initialization logic circuitry 111 (hereinafter referred to as IL logic circuit 111) to interval driver logic circuit 113 (hereinafler referred to as ID logic circuit l 13);

ii. terminal 1100 connects to the logic common (LC) of the DFS; and

iii. terminal 110d passes clock pulses at a certain rate, e.g., 400 112., through start/stop/restart logic circuitry 114 (hereinafter referred to as SSR logic circuit 114), to the following: to IL logic circuit 111, to timing circuitry 115 and to ID logic circuitry 113.

Even though the source code is present at terminals 1100-1 10d does not, however, necessarily mean that a shooting cycle will be initiated. Independent control of such operations is also provided within the shooting truck by means of start/stop/restart switch control 116 (hereinafter called SSR switch control 1 16) connected to SSR logic circuit 114 and to IL circuit 111 as shown. Should the operator with to stop, for example, the shooting operations at a selected point and, thereafter, continue such operations, i.e., stop then restart, he can activate switches of SSR switch control 116 (usually called stop and restart switches) to efi'ect such control operations in the SSR logic circuit 114 without losing either record counting control, as at record count and cycle control logic circuit 117 (called RC logic circuit 117 hereinafter) or sequence control at reset logic network 118. Similarly, if retention of the prior performed shooting functions is not needed, the operator can close other switching elements within the SSR switch control 116 (usually called a reset switch) to control operation of initialization logic circuit 1 11 to begin the operations anew, at time zero.

Output terminals 119a and 119b of ID logic current 113 are used to pass control signals generated within time decoder 105 to relays K, and K of FIG. 5. The duration of these control signals is not fixed by the source code, however, but can be independently changed by the shooting truck operator by activation of switch networks within timing counter 115 and record counter 117, as explained below. Since a selected number of shooting cycles, up to 32, are to be collected, automatically, before the entire operation is interrupted, the RC logic circuit 1 17 is provided with controls to automatically initiate new shooting cycles; when a preselected number of records have been collected, it then terminates operations. Binary record indicator 120 allows the operator to visually observe the record counting operation.

Individual elements of the time decoder depicted in FIG. 8 have been constructed and tested successfully, and will now be described, in detail, with reference to FIGS. 9, 10, 11, 12, 13 and 14. In the description that follows the initials FF stand for flipflop circuits.

INITIALIZATION LOGIC CIRCUIT l 1 1 (FIGS. 90 and 9b) As shown, input terminals 121a and 121b connect by conductors to bistable F F 123, 124 to pass word pulses and clock pulses respectively through these FF 123 and 124 to an output network indicated at 127a-l27d at selected time sequences. In more detail, a word pulse passes through terminal 121a to bistable FF 123 which provides a reset signal at output gate 126 and terminal 127a. Terminal 127a, in turn, as detailed below, connects to reset logic circuit 118 of FIG. 8. After the reset signal has passed to the reset logic circuit 118, as explained below, not only is that circuit reset to zero, but it also provides, as explained below, a reset pulse for bistable FF 123 and 124 passing thereto by way of input terminal l2lc. Input terminal 121b is seen to be connected to FF 123. Clock pulses pass through the input terminal l2lb to FF 123 which then triggers, by way of conductor 123a, FF 124. The FF 124 provides in conjunction with gates 129 and 130 word pulses (one and zero) at output terminals 127a and 127d which pass therefrom to switch network 112, FIG. 8. Gate 128 is likewise enabled by FF 124 for enabling an appropriate visual indicator at SSR switch control 116. At the end of each cycle, a restart pulse from ID logic circuit 113, as explained below, reinstates operations by passing the restart pulse through input terminal 121d to FF 123 for enabling gate 122.

Should the operator desire to abort operations, the stop switch in the SSR switch control 116 connected to the IL circuit 111 by way of terminal 127e, is opened disabling the gates 126, 129 and 130. To restart operations at the beginning of the cycle (time-zero) the restart switch within the SSR switch control 1 16 can be closed by the operator as at input terminals 125a and l25b activating through gate 132, FF 132 and output terminal 133, the reset logic circuit 118, as explained below.

START/STOP/RESTART LOGIC CIRCUIT 114 (FIG. 10)

Assuming that the operator at the shooting truck has pre-programmed the shooting operations as to number and duration of the sequence steps (as determined by switch conditions at timing counter 115) as well as to the repetition rate or number of records to be made without interruption (as determined by switch conditions at record counter 117), the SSR logic circuit 114 is initialized; reset pulses pass through input terminals 135!) and 135f to FF 138, 139, and 151, 152 respectively from reset logic circuit 118. In general, the functional operations of SSR logic circuit 114 include the provision of initiation of signals at output terminals 144a-144g, based on commands initiated at either the recording or shooting truck, or at both. Now in more detail, assume that clock pulses have enabled gates 147-149. At input terminal 135a, also assume that closure of the start switch within SSR switch control 116 has occurred, which triggers FF 138 and 139 to provide pulses at output terminals 1440-1440. Signals at output terminals 144a-144c function as follows:

Activated In order to allow the operator to manually interrupt operations, but then restart operation, without losing either record or sequence count, the input terminal 135d of SSR logic circuit 114 is connected to the stop switch at SSR switch control 1 16, as shown in FIG. 8. It is recognized that such stop operation could be automatically generated using relay 89 (FIG. 7) to close the stop switch at SSR switch control 116. Input terminal 135e connects to the restart switch of SSR switch control 116.

Activates Item Through Function of Signal (1) Output- FFlSl, Gate Disables Gate 140 so as Terminal 150 FF 152, to disable enabling pull35d Gates 153, ses at output terminals (STOP) 155 144a-l44c: also produces ZERO signal at output terminal 144g for resetting of reset logic circuit 118 which in turn resets ID logic circuit 113 viz. at input terminals 171 f and 180f(FIGS. 12a & 12b). Also enables gate 155 at output terminal 144f to light a stop bulb at SSR switch control 116.

(2) Input Gate 146, Enables gate 140; produces Terminal FF 151, a ONE signal, at output 1352 152 Gate terminal 144g disabling (RESTART) 153, Gate gate 155; at output ter- 150, Gate minal 144a produces a ZERO 155, Gate signal for incrementing I40 the RC logic counter 1 17 to a new value at input terminal 205d thereof (FIG. 14).

At output terminal 144k, clock pulses pass to timing counter 115, ID logic circuit 113 and IL circuit 111.

At the end of each sequence, the operation can be automatically repeated as explained below with reference to RC logic circuit 117 (FIG. 14) activating reset logic circuit 118 (FIG. 13) to generate signals at input terminals 135b and 135f for resetting FF 138, 139 and 151, 152 of SSR logic circuit 114. Prior to resetting of the F Fs, a signal for FF 152 at input terminal 1350 is generated from ID logic circuit 113 (FIG. 12b), viz. 212;" output terminal 18lb thereof (FIG. 126) to set FF TIMING CIRCUIT (FIG. 11)

Timing circuit 1 15 is depicted in detail in FIG. 1 1.

As shown, after the circuit 115 has been initialized, clock and start signals (ONES) appear by way of input terminals a and 16% to trigger FF 162, 163 through gate 167. The start signal passes through input terminal 160a from SSR logic circuit 114, viz. from output terminal 14417 (FIG. 10), while the clock pulses (400 Hz.) from the same SSR logic circuit 114 appear at input terminal 16012. The output of FF 162 is a multiple of the clock pulses, say 200 I-Iz., this output appearing at output terminal 166a for transference to ID logic circuit 113. Similarly, the clock pulse output of FF 163, say 100 Hz. appears:

I. at terminal l66b for transference to two separate circuits i. to the ID logic circuit 113, as explained above,

and

ii. to record counter (RC) logic circuit 1 l7 and 2. at counters 164, 165.

However, item (ii) relates only to the fact that initialization of all circuitry must occur before the operating cycle, viz. item (i) can occur.

In this regard, the initialization occurs by the triggering of a one-shot multi vibrator in record counter (RC) logic circuit 117, which, in turn, causes reset logic circuit 1 18 to be activated to produce selected reset functions to start the process.

Counters 164, are conventional, accumulating clock pulses. After a selected number of pulses are accumulated, under control of switch network 168 and gate network 167, a series of control signals are produced at output terminals 169c and 169d. These control signals functionally control the ID logic circuitry 113. Resetting of the counters 164, 165, as well as FF 162, 163 at the end of each cycle is by means of a signal (ONE) passing through input terminal 160C and gate 168. The-reset pulse is generated by reset logic circuit 118.

INTERVAL DRIVER LOGIC CIRCUIT 1 13 (FIGS. 12a & 12b) Interval driver logic circuit 113 (FIGS. 12a and 12b) is shown in detail in FIGS. 12a and 12b.

In FIG. 12a, the circuit is seen to provide a timing control signal at output terminal 170 for activation of relay K of FIG. 7 for a predetermined interval (called the AT timing interval) so as to control the amount of explosive gas downhole in the gun. In order to activate the gates 172, 173 in correct timed sequence, terminal pairs 171a, 17lb and 171C and 171d, each must pass identical coded pulses. For this purpose, terminals 171a and 1710 connect to terminals 169a and 16% of timing counter 115 (FIG. 10). As timing sequences are generated, they pass through gates 172-177 to FFS 179 and FF 178. Simultaneously, terminals 171b and 171d provide passage of timing word pulses from IL logic 1 11 through switch network 1 12 (FIG. 8).

A start pulse (or aiternately, a stop pulse) from SSR logic circuit 114 passes by way of input terminal 17le to gates 176, 177 while a restart pulse (initiated when the operator, for example, utilizes SSR switch control 116 of FIG. 8) appears at terminal l71f. That reset pulse is generated at reset logic 118 and resets FF 178. 

1. A repetitive seismic source system comprising: a. an elongated pipe adapted for support within a borehole penetrating an earth formation at least partially filled with liquid: b. an elongated enlarged firing chamber attached, at one end, to a remote end of said pipe within said borehole and open at the other end so as to contact said liquid in said borehole and to form an impact surface for said chamber; c. lateral offset means connected to a near uphole end of said pipe; d. means connected to lateral offset means for delivering to said chamber through said lateral offset means and said pipe, a combustible gas mixture of a predetermined amount; e. ignitor means mounted in said offset tube means; f. means electrically connected with said ignitor means for energizing said ignitor means to initiate combustion of said mixture whereupon a combustion wave travels through said pipe through said firing chamber and strikes said impact surface to said liquid to generate a seismic disturbance; g. length/diameter ratio of said pipe being within a range to support said wave at supersonic speed; h. said firing chamber having a tapered end section connected to said pipe to re-establish said combustion wave therein after emergence from said pipe; i. time break indicating means attached to said firing chamber to indicate the occurrence of the passing of said combustion wave relative to said chamber; j. additional digital data responsive control means connected to parallel with said delivery means and said energization means connected to a time code generating means remotely positioned to said additional digital data responsive control means, for generating, in sequence, in accordance with a digital time source code, control signals for sequentially (1) controlling said delivery means to cause delivery to said elongated pipe and said elongated enlarged firing chamber of said predetermined amount of gas, and (2) actuating said energization means so as to energize said ignitor means, said digital source code constituting digital signals representative of multi-bit digital data timing words and a plurality of clock pulses, said remotely positioned time code generating means including gate means responses to said time break indicaTor means whereby said gate means is held in the enabled state for a predetermined duration to ensure detection of all significant seismic reflection signals after generation of said seismic disturbance, said additional digital data responsive control means including time decoding means for decoding said digital source code, first relay means electrically connected to said time decoding means and operative thereby to control said delivery means, so as to cause delivery along said elongated pipe and said firing chamber of said predetermined amount of said gas mixture, second relay means electrically connected to said decoding means and operated to control and cause actuation of said energization means in accordance with the occurrence of an operational coded event, said time decoding means of said additional digital data control means including: k. control means comprising start/stop/restart signal generating means and switch network means controllably connected thereto, said control means capable of generating digital signals for initiating, stopping and restarting delivery of gas and energization thereof within said system in accordance with the status of said switch network means; l. timing counter means connected to control means (k) and, including additional switch network means, said timing counter means accumulating clock pulses from a source of said pulses to provide a plurality of digital signals in accordance with switch conditions of said additional switch network means; m. control signal generating means including gate means selectively responsive to digital signals from control means (k) and timing counter means (l) as well as digital signals indicative of a multi-bit digital data timing word passing through control means (k) for generating at least first and second groups of control signals in which said first group sequentially controls said first and second relay means and hence the occurrence and duration of delivery as well as ignition of said gas within said pipe and chamber; n. cyclic record counting means including yet additional switch network means and gate and counter means responsive to digital signals from control means (k) which indicate each shooting cycle and compare the indicated cycle to an encoded ending cycle value in accordance with conditions of said yet additional switch network means, said record counting means also including a digital signal generating means responsive to digital signals from said gate and counter means thereof as well as to said second group of control means generated by control signal generating means (m) to provide a cycle reinitiation digital signal; o. reset logic means including a series of gate means responsive to at least said cycle re-initiation digital signal from record counting means (n) to provide a series of set and reset digital signals for circuitry (k)-(n) for setting said (k)-(n) circuitry to preselected initial conditions whereby the seismic shooting cycle can be automatically repeated without interruption until said ending cycle value encoded in record counting means (n) is attained.
 2. The system of claim 1 in which said temperature conditional interrupt circuit means includes additional interruption means for controllably interrupting said switch network means of control means (k) of said additional digital response control means.
 3. The system of claim 1 in which said switch network means of control means (k) includes first separate switch conditions means for controlling starting, stopping and restarting of said signal generating means to provide delivery or ignition of gas within said pipe without repeating previously performed functions, and second separate switch condition means for dumping previously performed functions and restarting the system at time zero.
 4. The system of claim 1 with the addition of a temperature conditional interrupt means operative at said lateral offset means to detect the occurrence of a selected high temperature conditioN therein and to generate a control signal in response thereto for disconnectably connecting said delivery means from said pipe to interrupt operations thereof independent of said additional digital response control means. 